1. Field of the Invention
This invention relates generally to transmit circuit architecture in a wireless portable communication device. More particularly, the invention relates to a software defined multiple transmit architecture for a wireless transmitter that is capable of communicating using multiple transmit methodologies.
2. Related Art
With the increasing availability of efficient, low cost electronic modules, mobile communication systems are becoming more and more widespread. For example, there are many variations of communication schemes in which various frequencies, transmission schemes, modulation techniques and communication protocols are used to provide two-way voice and data communications in a handheld, telephone-like communication handset. The different modulation and transmission schemes each have advantages and disadvantages.
As these mobile communication systems have been developed and deployed, many different standards, to which these systems must conform, have evolved. For example, in the United States, third generation portable communications systems comply with the IS-136 standard, which requires the use of a particular modulation scheme and access format. In the case of IS-136, the modulation scheme can be 8-quadrature phase shift keying (8QPSK), offset π/4 differential quadrature phase shift keying (π/4-DQPSK) or variations thereof and the access format is TDMA.
In Europe, the global system for mobile communications (GSM) standard requires the use of the gaussian minimum shift keying (GMSK) modulation scheme in a narrow band TDMA access environment, which uses a constant envelope modulation methodology.
Furthermore, in a typical GSM mobile communication system using narrow band TDMA technology, a GMSK modulation scheme supplies a very low noise phase modulated (PM) transmit signal to a non-linear power amplifier directly from an oscillator. In such an arrangement, a non-linear power amplifier, which is highly efficient, can be used thus allowing efficient modulation of the phase-modulated signal and minimizing power consumption. Because the modulated signal is supplied directly from an oscillator, the need for filtering, either before or after the power amplifier, is minimized. Further, the output in a GSM transceiver is a constant envelope (i.e., a non time-varying signal containing only a phase modulated (PM) signal) modulation signal.
Many non-constant envelope transmit architectures use a modulation scheme where both a PM signal and an amplitude modulated (AM) signal are transmitted. Standards employing these schemes increase the data rate without increasing the bandwidth of the transmitted signal. Unfortunately, even though it would be desirable to have one portable transceiver that can accommodate all of the above-mentioned transmission schemes, existing GSM modulation schemes are not easily adapted to transmit a signal that includes both a PM component and an AM component. One reason for this difficulty is that in order to transmit a distortion free signal containing a PM component and an AM component, a highly linear power amplifier is required. Unfortunately, highly linear power amplifiers are very inefficient, thus consuming significantly more power than a non-linear power amplifier and drastically reducing the life of the battery or other power source.
In non-constant envelope modulation, an amplitude modulated (AM) portion of the signal causes the transmit output signal to vary in amplitude. In constant envelope modulation, the transmit output signal is always at a constant amplitude. Emerging communication standards, such as enhanced data rates for GSM evolution (EDGE), which is an extension to the global system for mobile, communications (GSM) and wide band code division multiple access (WCDMA) will likely use a non-constant envelope modulation scheme. As the transmit architectures for these new standards are under development, it is generally desirable to have a single transmit architecture that supports as many standards as possible.
One possible manner of developing a single transmit architecture that is capable of both constant envelope and non-constant envelope modulation use a conventional upconverter with filters inserted into the transmit chain. Such an architecture requires filters at the intermediate frequency (IF), at the radio frequency (RF) before the power amplifier and at RF after the power amplifier. Unfortunately, a multi-standard transmit architecture would require many filters to be switched in and out of the transmit circuit, or would require separate transmit chains.
Further, when a new wireless communication system standard emerges, it is desirable to have a transmit architecture that can satisfy the new standard and still remain “backwards compatible” with existing standards. Often the new standard is designed using principles and techniques that are fundamentally at odds with one another. For example, an existing standard may us TDMA, narrow signal bandwidth, constant-envelope modulation (i.e., GSM), while a new standard may use code access (CDMA), wide signal bandwidth, non-constant envelope modulation (i.e., wideband CDMA (WCDMA)). Further, the two systems may operate in different frequency bands. Such differences in transmission standards cause a “ripple effect” throughout the system design process and will typically result in very different transmit architectures (i.e., different filtering, different power amplification, etc.). Yet, it is desirable to have a single transmit architecture that can satisfy multiple standards.
Existing transmit architectures for wireless digital standards are not sufficiently flexible to support alternative standards without significant modifications of the transmit hardware. As a result, existing multiple standard (also referred to as “multi-band” or “multi-mode”) transmit architectures require a significant number of components including both active and passive devices. This can require that a single portable communication device include two separate transmitters, resulting in a costly and excessively bulky device. Further, when operating in communication standards that require non-constant envelope modulation (i.e., standards that include an AM component), power efficiency is reduced, resulting in shortened battery life and increased heat dissipation from the active components. An example of such a situation is the universal mobile telephone service (UMTS) standard, which requires a portable communication device to operate in both GSM mode and WCDMA mode.
With the increasing desirability of developing one worldwide portable communication standard, it would be desirable to have a portable transceiver that can operate in multiple digital standards, while minimizing the number of components in the transmit architecture.